1. Field of the Invention
The present invention concerns a process for the purification of wet phosphoric acid and, more specifically, it concerns a process for removing fluorine from same, thus affording a phosphoric acid of food quality.
2. Discussion of the Prior Art
It is known that the attack of an acid on natural phosphate rock yields after filtration a raw phosphoric liquor which contains numerous anionic and cationic impurities, including metallic ions such as the ions of Al and Fe, as well as anions such as those of the attacking acid, fluoride, silicofluoride originating in the impurities of the rock, etc. This impure phosphoric acid is unsuitable for certain applications and for this reason various purification processes have been proposed. In particular, the presence of fluorine in any form--namely, in the form of a fluoride ion, a fluosilicic ion or a metal-fluorine complex--prevents its use in the food industry, where it is specifically required that phosphoric acid must have a fluorine percentage relative to the percentage of P.sub.2 O.sub.5 in the aqueous solution, below 10 parts per million (ppm).
Various processes for the elimination of fluorine have been proposed, consisting particularly of precipitating the fluorine from the raw phosphoric liquor in the form of the fluosilicate of an alkali metal or an alkaline earth metal. Another type of process consists of entraining the fluorine from the raw acid with water vapor (steam) or a hot gas, in the form of a volatile species (HF or SiF.sub.4). These processes yield at best a phosphoric acid with a F/P.sub.2 O.sub.5 content of 500 ppm, which again renders it unsuitable for use in the food industry.
In U.S. Pat. No. 3,607,029, issued Sept. 21, 1971, there is described a process for the purification of a raw aqueous acid resulting from a wet process comprising, in its most general aspect, the extraction of the raw acid by means of an alkyl phosphate in the presence of a strong acid, followed by the regeneration of the phosphoric acid with the aid of water, to obtain as the result, an aqueous solution of phosphoric acid substantially free of cationic impurities, but still containing a F/P.sub.2 O.sub.5 content of up to 7000 ppm, which again renders the acid unfit for use in the food industry. The subject matter of U.S. Pat. No. 3,607,029 is hereby incorporated by reference into the present specification and relied upon.